1. Technical Field
The present invention relates to a technology contributing to simplifying a configuration of an electro-optical device.
2. Related Art
Recently, projectors for forming a reduced image by the use of a display panel employing liquid crystal and enlarging and projecting the reduced image through an optical system are becoming increasing popular. The liquid crystal is alternately driven with a positive polarity and a negative polarity in principle, so as to prevent deterioration of the liquid crystal. In case of such alternate driving, the following 4 methods are considered to control writing polarities of pixels in a screen:
(1) scanning line inversion in which the writing polarity is inverted every scanning line (line inversion);
(2) data line inversion in which the writing polarity is inverted every data line (source inversion);
(3) pixel inversion in which the scanning line inversion and the data line inversion are combined and the writing polarity is inverted between the pixels adjacent to each other in all directions (dot inversion); and
(4) surface inversion in which the writing polarity of a screen is inverted (frame inversion).
On the other hand, in any case of (1) to (4), the writing polarity is inverted with an interval of one or more vertical scanning period of time (frame).
In the scanning line inversion of (1), the data line inversion of (2), and the dot inversion of (3), the polarities of a pixel row and/or a pixel column spatially adjacent to each other are changed. Accordingly, even when the effective voltage values applied to the liquid crystal is different in polarity, the flickering resulting from the difference thereof is little recognized.
However, since gaps between the pixel electrodes are very small in a display panel on which the above-mentioned reduced image is displayed, a disclination (alignment failure) due to a so-called lateral electric field occurs in (1), (2), and (3). Accordingly, the surface inversion of (4) is effective when the gaps between the pixel electrodes are very small.
In the surface inversion of (4), when the inversion cycle is one vertical scanning period and attention is paid to the data lines in a column, the data signals having the same polarity are written to a column of pixels supplied through the corresponding data lines with the data signals in one vertical scanning period and the polarity of the data signals supplied to the data lines is inverted in the next vertical scanning period.
As a result, when the scanning lines are scanned from the upside to the downside in a display area, the data signal supplied to the data line of the relevant column is changed to the same polarity as that of the data signal written to the upper pixel in most of the non-selected period as seen from the upper pixel corresponding to the intersection between the scanning line located upside and the data line in the relevant column. However, the data signal supplied to the data line of the relevant column is changed to the polarity opposite to that of the data signal written to the lower pixel in most of the non-selected period as seen from the lower pixel corresponding to the intersection between the scanning line located downside and the data line in the relevant column.
Therefore, in the upper pixel and the lower pixel, the voltage of the data line in the sustain period differently affects the pixel electrodes, thereby making the display quality non-uniform depending upon positions on a screen.
On the other hand, there has been suggested a technology of setting the polarity of a data signal supplied to a data lines to positive and negative by 50%, by virtually (not physically) dividing a screen into an upper half and a lower half, alternately selecting a scanning line in the upper half and a scanning line in the lower half in a predetermined order, writing the data signal with one of a positive polarity and a negative polarity when the scanning line in the upper half is selected, and writing the data signal with the other of a positive polarity and a negative polarity when the scanning line in the lower half is selected (see JP-A-2004-177930).
However, in the technology, for example, after a data signal of a gray scale with a positive polarity is written to a pixel row, the data signal of the same gray scale with a negative polarity has to be written again to the pixel row. Accordingly, in the technology, since the image data supplied from the outside has to be stored in a memory and the image data supplied from the outside and the image data read out of the memory have to be alternately supplied every horizontal scanning period, there is a problem in that the configuration is complex.